Okay, this is what I would do. I can't say for sure that any of this will help, but it sure can't hurt. Do a water change, then do another one 2 days later. Make sure that at least one powerhead is aimed near the surface to provide agitation. Turn your lighting off for a few days (if possible......if you can't because of corals, then darken one side underneath the lights. You can use a dark plastic or wood, anything that is not flammable, NO PAPER). Watch him closely for any signs of white spots, like grains of salt, or a white powdery dusting on him, or a change of breathing. Do not attempt to feed him right now. Don't panic.
Good luck......